1. Field of the Invention
This invention relates generally to high-vacuum pumps, and more particularly to a high-vacuum pump which can continuously achieve an exhaust or pumping operation over a wide vacuum range.
2. Description of the Prior Art
Conventionally, the oil-sealed rotary pump of the Gaede type has heretofore been utilized in which, as the rotor advances, the inhalation, compression, and exhaustion stages of air or other gas are cylically repeated so as to achieve the exhaust operation of the air. The above rotary pumps are well known in the art as being effective to obtain pressures of 10.sup.-4 Torr or less as ultimate pressures. In other words, the rotary pumps are most effective in the viscous flow region of air.
The molecular pump such as that of the Siegbahn type has also been proposed in which air or another gas is compressed and exhausted along grooves when a circular disk rotates inside a metal container consisting of two shields having the grooves. The molecular pump has been considered to have many advantages over diffusion pumps. Firstly, the time required to get the pumps into generation is less than that required for diffusion pumps. Secondly, such pumps pump all kinds of gases and vapors which means that no liquid-air traps or similar devices are required. Lastly, molecular pumps have the property of pumping heavy gases faster than light ones in contrast to diffusion pumps which behave in the opposite manner.
Therefore, it will be considered that in order to obtain a vacuum over a wide range extending from atmospheric to a high-vacuum of approximately to.sup.-6 Torr, the oil-sealed rotary pumps which now function as backing pumps may be combined with the molecular pumps of the Siegbahn type which are effective in the molecular region of air.
However, in the molecular pumps of the Siegbahn type having grooves, a high compression ratio of 10.sup.5 or more is required because such pumps have been designed and intended to obtain an inlet vacuum pressure of a 10.sup.-6 or 10.sup.-7 Torr level under forepressure of a 10.sup.-1 or 10.sup.-2 Torr level. Therefore, the depth of the grooves should be considerably shallow in view of the mean free path of the molecular exhausted air and the depth thereof near the outlet of the pumps will usually be one millimeter. This results in the fact that the continuous pump operation under 0.5 or 10 Torr produces a considerably high amount of heat, and this disadvantage is apt to occur wherein the disk is brought into contact with the shields due to a thermal expansion thereof.
In the above construction, if the outlet of the molecular pump of the groove type is connected in series to the inlet of the rotary pump so as to achieve a two-stage, successive pumping function from atmospheric to a high-vacuum level, the molecular pump acts as the exhaust resistance against the rotary pump when in the viscous flow region of air, so that the speed of the exhaust of the rotary pump is decreased.
In addition to the above requirement relating to the high compression ratio, the clearances between the sides of the circular disk and the shields have to be several hundredths of a millimeter in order to maintain the fore pressure, and spiral grooves must be cut in the shields, the same being deep at the periphery and gradually decreasing in depth towards the center. Due to the aforenoted small clearances, it is possible that foreign objects may be trapped in the clearances, and the disk may encroach upon the shields because of a partial thermal expansion.
In addition, in the molecular pumps of the groove type, the speed of the exhaust is in proportion to the cross-sectional areas of the grooves and the velocity of the disk. However, it is quite difficult to design grooves of increasing depth or to increase the number of the grooves, due to the aforenoted requirement of a high compression ratio.
Another disadvantage of prior art molecular pumps of the spiral or screw groove type is that the bearings for the drive shaft are situated in the region of the fore-vacuum, and the vacuum seal mechanism, such as the packings for the drive shaft, are also situated in the region of the fore-vacuum. This means that lubricants for the bearings are subjected to vacuum pressure which results in a decrease of the durability of the bearings and the seal mechanisms, and in an increase in vacuum leakage, especially when there is high rotation of the drive shaft. In other words, it is substantially impossible to maintain a fore-vacuum of 10.sup.-2 Torr or more when there is high rotation of the shaft, such as, for example, at 6,000 R.P.M., or more.
The aforenoted varius requirements and drawbacks result in the impracticability of molecular pumps. It has been theoretically considered to design a high vacuum pump mechanism wherein the fluid in the viscous flow region is firstly pumped out to the fluid in the molecular flow region by means of the rotary pump assembly, and thereafter, the fluid in the molecular flow region is pumped out so as to obtain a high-vacuum level by means of the molecular pump assembly. However, this mechanism still does not achieve a sufficient pumping operation due to the aforenoted drawbacks of molecular pumps. In addition, the aforenoted mechanism requires two different driving means for each pump assembly and a by-pass passage means, for the rotary pump assembly, which is controlled by a change-over valve. These requirements result in a large-sized exhaust system and in great complexity in manipulating the apparatus.